Superior Acute Myeloid Leukemia-Specific T Cell Responses Using Dendritic Cells Pulsed with Apoptotic Bodies, vs.Tumor Lysates or mRNA.

Dendritic cells (DCs), which hold promise for cancer immunotherapy have two distinct functions. Immature DCs internalize antigens; while mature DCs trigger native T cell activation. To date, no consensus has been reached concerning the optimal source of tumor antigen nor the mode of antigen uptake for efficient cross-priming of tumor specific response. Reports have demonstrated that human DCs can acquire relevant antigens and stimulate MHC class I-restricted cytotoxic T lymphocytes (CTLs) by phagocytosing apoptotic tumor cells. Apoptosis was established to be the critical trigger of cross-priming in this process. Apoptotic bodies form when cells undergo programmed cell death. In this study, three acute myelogenous leukemia in (AML) antigen preparations were compared in an autologous setting.

1. Apoptotic bodies,

2. Tumor lysate

3. Tumor mRNA

Methods: DCs were generated by CD14 magnetic selection of peripheral blood progenitor cells (PBPCs) and six days of culture in GM-CSF and IL-4. Tumor lysate was made by repeated freezing and thawing of patient AML cells (leukodepletion product >95% blasts); mRNA was extracted using a kit (Qiagen); apoptotic bodies were generated with UVB irradiation of a fraction of the same AML product. The rate of apoptosis was detected using Annexin V FITC flow cytometry. The DCs were loaded with mRNA by electroporation. The DCs were loaded with lysate and apoptotic bodies by incubation. Autologous T cells stimulated with DCs loaded with the three differing antigen preparations were compared for interferon(INF)-g production in ELISPOT and proliferation assays.

Results: The DCs pulsed with apoptotic bodies; lysate and mRNA expressed similar levels of the costimulatory molecules CD80, CD86, and theDC maturation marker CD83. The internalization rate of apoptotic bodies was about 20% detected by DAPI (label the apoptotic bodies) and DC83 double staining. The lysate pulsed DC and mRNA pulsed DCs were above 90% detected by flow of trace FITC- albumin and tracer GFP mRNA from previous experiments. Yet IFN-g secretion of T lymphocytes was statistically significant higher (P<0.05), using apoptotic body-loaded DC as compared to total lysate or mRNA. The T cell proliferation stimulated by apoptotic body-loaded DC was also better than that of lysate or mRNA loaded DCs (p<0.05).

Conclusions: These preliminary data demonstrate that apoptotic body-loaded DCs induced better Th1 T cell responses than DCs loaded with AML lysate or mRNA. One potential advantage conferred by the use of apoptotic body loaded DCs is that presentation of intracellular proteins in tumor lysates and mRNA that are irrelevant to the induction of anti-tumor responses are eliminated, which may lead to better antigen presentation and consequently an improved Th1 response.